I can't put my finger on it now, but I seem to recall a MkI receiver used for another rifle believed to be assembled post-war - can anyone remember the thread?
Your blanket statement about "would not have used a MkI receiver to assemble anything after the war" cannot be proven, nor does it make sense IMHO. The MkI* modification was a foolish idea that the UK factories pointedly avoided. That should have been enough to convince LB and Savage not to proceed, but further research needs to be done on why they did. If I were having a rifle built there, or building one for someone I knew, you can be damn sure it would have a MkI receiver, if one was available.
Now, finding a Long Branch bolt head retaining catch is not an easy thing at all, but you can bet they had some at Long Branch in the drawers and boxes, and if this rifle was assembled there without one, that suggests to me that it was just a "looker", not a "user".
Alternatively, if some industrious person was assembling a rifle some time in the not-too-distant past, a Long Branch bolt head retaining catch would be a very hard thing to find, and if they were 'trying to put one over', using a Britishmade catch would be shall we say, "unconvincing"! Better perhaps to have no catch at all??
Or maybe someone was restoring a Long Branch MkI and needed a bolt head retaining catch and 'this here rifle' just happened to provide one?!
As I said, the only possible conclusion is that there can be no conclusion.